The present invention relates to a writing instrument of the type having a tubular writing tip and a needle longitudinally movably mounted within the tubular writing tip, and more particularly to such an instrument further comprising a weight or an inertia member connected to the displaceable needle so as to enhance the longitudinal movement of the needle for the principal purpose of ensuring a continuous and smooth capillary action of ink from an ink reservoir towards the tubular writing tip.
In a conventional writing instrument of the type described, an ink feeding action or capillary action towards the writing tip is enhanced by providing a longitudinally movable needle as described above, or by providing an ink carrier or a supplemental ink holder between the tubular writing tip and the ink reservoir.
In case of the movable needle, the longitudinal movement of the needle for enhancing the ink capillary action has been effected either by providing a longitudinal weight member at the rear end of the needle so that movement of the needle is enhanced by the inertia of the weight member, or by providing a resilient seat, such as an ink permeable resilient member as described in U.S. Pat. No. 4,364,684 assigned to the present assignee, or a spiral spring as suggested in U.S. Pat. No. 2,029,152. The movable needle functions to clean any foreign materials which block the tubular writing tip and also to provide a desirable capillary action in an annular space defined between the tubular writing tip and the surface of the needle.
In case of the supplemental ink carrier provided between the ink reservoir and the writing tip, the ink carrier carries an excessive ink temporarily so as to prevent ink from dropping from the writing tip, such ink drop generally being due to some change in the ink reservoir as a result of a change of atmospheric pressure and/or temperature, and to provide a continuous, smooth flow of ink to the writing tip to avoid any obstruction of ink flow or shortage of ink at the writing tip.
The needle and/or supplemental ink carrier are advantageous mechanisms, but they do not always work as well as expected and sometimes fail to work effectively. The inventors of the present invention consider that one of the reasons for failure of the expected function is that an air ring or bubble is retained at portions in the ink channel, particularly at a position adjacent the front end of the ink reservoir, adjacent a stopper element for limiting a backward movement of the weight element. Another reason is that the supplemental ink carrier, when used as an ink feed between a cylindrical tip holder, which is mounted on a casing of the instrument and longitudinally slidably holds therein the needle, and the ink reservoir, is deformed, though very slightly, during the step of assembly. This is especially true when the ink carrier is of the type having a number of annular small recesses to form a bellows-like structure.